(I don't know if the Italian- French chains still exist, but there are chains in Africa and among the Eskimo)
These, like ring species, are synchronic examples of the phenomenon. Latin to French (or Spanish or Italian) is a diachronic example of the same process; Old English to Modern likewise. In some cases there are written records but in others, forensic analysis gives the best guess.
It's similar for species formation. One can infer ancestors based on DNA, bone structure, etc. (Or synchronically one has goldfish vs carp, if one takes 1000 years as a short time biologically.)
There are a few examples of instant speciation, usually by polyploidy; the offspring can interbreed but cannot back breed to the ancestral species. This is often seen in plants but rarely in animals (maybe some frogs) because the polypoloid cells get too big.
True of Germany and China, at least before the movies/TV/radio age. The Deutsch of Berlin used to shade imperceptibly to the Dutch of Amsterdam to the west and the Scaninavian German languages to the north. China supposedly has several distinct dialects (Mandarin, Cantonese, Hakka ...) but apparently boundaries are not sharp. I have heard that from anywhere you could go one village in any direction and not notice much difference. Two villages and people seemed to be talking a bit strangely. Four or five villages and it was getting hard to communicate.
There is a global tendency now to standardize education around the speech of the capital. That's homogenizing a lot of the regional flavors.